Nodular iron (also known as nodular cast iron, ductile iron, spherulitic iron and spherulitic graphite iron) is cast iron in which the graphite is present in tiny balls, spheroids or spherulites instead of flakes (as in gray iron) or compacted aggregates (as in malleable iron). The spheroidal graphite structure is produced by the addition of one or more graphite-spheroidizing agent to molten iron. A number of elements (e.g., magnesium, cerium, calcium, lithium, sodium, barium) have been found to be suitable graphite-spheroidizing agents of which magnesium and cerium (and particularly magnesium) are commercially important.
The graphite-spheroidizing agents generally are quite oxidizable and have low boiling points compared with iron. A wide variety of processes and apparatuses have been developed to introduce a graphite-spheroidizing agent such as magnesium into molten cast iron. Many of these known techniques utilize relatively complicated, expensive and/or cumbersome equipment or compositions. Also, violent, uncontrolled reactions often occur with the introduction of magnesium into iron which reactions pose serious safety hazards.
A number of widely used methods include, for example, the use of a high strength ladle and cover which seal the molten iron while a ram forces magnesium (usually in alloy form) into the melt. Magnesium may also be placed in a ladle into which molten iron is rapidly poured. Often the magnesium is first covered by a sheet of iron or steel prior to addition of the molten iron.
In spite of various precautions attempted with such methods, the loss of magnesium by volatilization and/or combustion is considerable and, concomitantly, the reaction efficiency (measured in terms of magnesium recovery) is quite low. It has been found that magnesium recovery in the spheroidizing treatment generally is between 20 and 60 percent depending on the particular technique used. On the average, magnesium recovery in commercial spheroidizing treatments is about 38 percent.
The search for more efficient processes for producing nodular iron has continued.
Often, nodular iron is contacted just prior to casting (i.e., within 15, often within 10, minutes before casting) with an inoculant alloy, usually a ferrosilicon alloy, which additionally promotes graphitization in the iron. That is, the first (or "spheroidizing") treatment may be sufficient to completely spheroidize all of the graphite present but it may be insufficient to graphitize all of the carbon present. The inoculant treatment promotes graphitization, governs the amount of graphite formed and also may be used to introduce small amounts of alloying elements into the nodular iron.
U.S. Pat. No. 3,033,676, for example, discloses an inoculant alloy which is primarily useful as a graphitizing inoculant in the treatment of foundry gray irons to produce gray iron containing randomly distributed flakes or flake graphite type A. This inoculant alloy (which broadly includes 0.1 to 5 percent magnesium, 0.1 to 10 percent aluminum, 0.1 to 60 percent nickel or iron or both and 15 to 99.6 percent silicon) is also suggested for use in the production of nodular iron either as a substitute for the graphite-spheroidizing agent or in addition and subsequent thereto. Suitable alloys disclosed therein include 2 or 3 percent magnesium.
It has been found, however, that the use of an inoculant alloy containing 2 or 3 more percent magnesium with nodular iron prior to casting of the nodular iron is accompanied by a violent, hazardous reaction of the same type that occurs in the spheroidizing of casting iron with magnesium. In view of the health and safety hazards created thereby (particularly in view of the stringent requirements of the recently adopted Occupational Safety and Health Act), the use of such magnesium-containing inoculants has been essentially stopped. Instead, inoculation prior to casting is typically conducted with a non-magnesium-containing ferrosilicon alloy.